Slope-Assisted Brillouin Optical
Correlation-Domain Reflectometry:
Proof of Concept
Volume 8, Number 3, June 2016
Heeyoung Lee
Neisei Hayashi
Yosuke Mizuno, Member, IEEE
Kentaro Nakamura, Member, IEEE

DOI: 10.1109/JPHOT.2016.2562512
1943-0655 Ó 2016 IEEEIEEE Photonics Journal

SA-BOCDR: Proof of Concept

Slope-Assisted Brillouin Optical
Correlation-Domain Reflectometry:
Proof of Concept
Heeyoung Lee,1 Neisei Hayashi,2 Yosuke Mizuno,1 Member, IEEE, andKentaro Nakamura,1 Member, IEEE
1

Laboratory for Future Interdisciplinary Research of Science and Technology, Tokyo Institute ofTechnology, Yokohama 226-8503, Japan
2
Research Center for Advanced Science and Technology, University of Tokyo, Tokyo
153-8904, Japan

DOI: 10.1109/JPHOT.2016.2562512
1943-0655 Ó 2016 IEEE. Translations and content mining are permitted for academic research only.
Personal use is also permitted, but republication/redistribution requires IEEE permission.
See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

Manuscript received April 27, 2016; accepted April 29, 2016. Date of publication May 3, 2016;
date of current version May 13, 2016. This work was supported in part by the Japan Society for
the Promotion of Science, Grants-in-Aid for Scientific Research (KAKENHI) under Grant 25709032,
Grant 26630180, and Grant 25007652; by the Iwatani Naoji Foundation; by the SCAT Foundation;and by the Konica Minolta Science and Technology Foundation. Corresponding author: H. Lee (e-mail:hylee@sonic.pi.titech.ac.jp).

Abstract: Exploiting the slope of the Brillouin gain spectrum, we develop a new configuration of Brillouin optical correlation-domain reflectometry, which can measure strain (ortemperature) and optical loss distributions simultaneously with a high sampling rate. The
strain, temperature, and loss dependence coefficients of the output signal are measured
to be 1:95  104 dB=", 4:42  103 dB/K, and 0.191, respectively, which are consistent with the theoretical predictions. We also verify the basic operation of simultaneous
measurement of the three parameters.
Index Terms: Brillouin scattering, optical fiber sensors, distributed measurement, correlationdomain techniques.

1. Introduction
To monitor the conditions of civil infrastructures, a variety of fiber-optic distributed strain andtemperature sensing techniques based on Brillouin scattering [1] have been extensively studied.
They are categorized into two types: “analysis” systems [2]–[19], in which two light beams need
to be injected into both ends of a fiber under test (FUT); and “reflectometry” systems [20]–[29],in which only one light beam is injected into one end of the FUT. The former include Brillouin optical time-, frequency-, and correlation-domain analysis (BOTDA [2]–[10], BOFDA [11]–[13], and
BOCDA [14]–[19]) systems, while the latter include Brillouin optical time- and correlation-domainreflectometry (BOTDR [20]–[22] and BOCDR [23]–[29]) systems. In analysis systems, stimulated
Brillouin scattering with a relatively high reflectivity can be exploited, leading to a high signal-tonoise ratio (SNR) of the measurement. Consequently, extremely high sensing performances,such as an ultimately fast measurement speed in BOTDA [8] and a nominal spatial resolution ofas high as 1.6 mm in BOCDA [17], have been reported. However, such two-end-access systems
reduce the degree of freedom in embedding the FUT into structures, and they do not function
properly when a single breakage occurs in the FUT. From the viewpoint of the users’Vol. 8, No. 3, June 2016

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SA-BOCDR: Proof of Concept

Fig. 1. Schematics of the operating principle of the slope-assisted Brillouin optical correlation-domainreflectometry (SA-BOCDR). (a) One-to-one correspondence between the BFS and the spectral power
PB0 at frequency B0 . The dotted curve is the initial BGS, and the solid curve is the BGS shifted to ahigher frequency on account of strain and/or heat. (b) PB0 distributions along the sensing fiber (solid
curve) with and (dotted line) without partial strain and heat. (c) PB0 distributions along the sensing fiber(solid curve) with and (dotted line) without a point loss.

convenience, one-end-access reflectometry systems are preferable. Compared to BOTDR,
where optical pulses are utilized to resolve the positions, continuous-wave-based BOCDR canachieve a relatively high SNR in principle, leading to a high spatial resolution and a sampling
rate.
BOCDR operates based on the correlation control of propagating lightwaves. Even when opticalloss occurs in the FUT, conventional BOCDR can provide the Brillouin frequency shift (BFS) distribution properly, which is derived from the Brillouin gain spectrum (BGS) distribution. This feature isan advantage from the aspect of stable strain and temperature measurement; however, in some
practical applications, optical loss distribution should be simultaneously obtained.
In this work, to achieve simultaneous distributed measurement of strain (or temperature) andloss, we develop a new BOCDR configuration—named slope-assisted BOCDR (SA-BOCDR) in
analogy with SA-BOTDA [5]–[7]—which operates with the assistance of the BGS slope. In thismethod, the whole BGS need not be observed to derive the BFS value, which potentially leads
to a higher sampling (or repetition) rate in principle.

2. Principle
As detailed in [1], BFS in an optical fiber shows strain and temperature dependences, which areoften exploited to implement fiber-optic sensors. One of such distributed strain/temperaturesensing techniques is BOCDR, which has intrinsic one-end accessibility, a high spatial resolution, a high sampling rate, and cost efficiency. To spatially resolve the sensing locations, we apply sinusoidal frequency modulation to the laser output to generate a so-called “correlationpeak” in the FUT [14]. Using the correlation peak, the BFS at a specific position can be selectively detected. By sweeping the modulation frequency fm , the correlation peak is scanned along
the FUT, enabling a distributed BFS measurement. As sinusoidal frequency modulation periodically generates multiple correlation peaks along the FUT, their interval determines the measurement range dm as [25]
dm ¼

c
2nfm

(1)

where c is the velocity of light in vacuum, and n is the refractive index of the fiber core. When fm islower than the Brillouin bandwidth B , the spatial resolution z is reported to be given by [25]
z ¼

cvB
2nfm f(2)

where f is the modulation amplitude of the optical frequency.
In conventional BOCDR systems, the BFS (i.e., strain or temperature) at one sensing point
is derived after obtaining the whole BGS. In contrast, SA-BOCDR provides the strain/temperature information using the BGS slope. This scheme has been implemented for BOTDA

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Fig. 2. (a) Measured BGS and (b) its differential coefficient at each frequency. The linear region of
the lower frequency side is indicated.

systems [5]–[7]. As depicted in Fig. 1(a), the BFS is in one-to-one correspondence with thespectral power PB0 at a certain frequency B0 , which is set at the high-frequency point in thelinear region (lower frequency side) of the BGS slope (see the following paragraph for a quantitative discussion). Then, when the BFS slightly shifts to higher frequency responding to strain
and/or heat, PB0 decreases linearly; when the slight loss occurs in the FUT, the spectral powerof the BGS entirely decreases, also leading to the reduction of PB0 . Even if the BFS change is
so large that B0 gets out of the linear region, as long as PB0 is in one-to-one correspondencewith the BFS, this system operates properly (with a reduced sensitivity) by simple nonlinear
compensation. Fig. 1(b) and (c) schematically shows the changes in the PB0 distributions
when strain (or temperature change) and loss are locally applied, respectively. The strain andtemperature effects cannot be separated in this method, but the loss effect can be discriminated from the locally applied strain (or temperature change) because the once decreased PB0
value does not return to the initial value (If strain (or temperature change) is applied from acertain point to the distal end of the FUT, it cannot be separated from the point loss; in whichcase, the problem could be resolved by utilizing the ratio of the spectral powers at two frequencies. The details will be published elsewhere). The PB0 change distributions [calculated
by substituting the resultant PB0 distributions (solid curves) from their initial distributions (dottedlines)] are used as final measurement data.
To evaluate the optimal B0 value and the bandwidth of the linear region, we analyzed a raw BGS
with a BFS of 10.89 GHz [see Fig. 2(a)], which was experimentally obtained when the FUT length L
and experimental conditions (such as the modulation frequency fm and amplitude f ) were set to thesame values as those for the experiment (L ¼ 5:0 m, fm ¼ 8:0 MHz, f ¼ 1:4 GHz; refer to the following sections). Here, we define the linear region as the region where the change in the BGS slope
is suppressed within 20% compared to its maximum. Based on the slope (i.e. differential coefficient)
dependence on the frequency [see Fig. 2(b)] obtained by differentiating the BGS with respect to frequency, the optimal B0 value to widen the linear region was found to be 10.85 GHz. The bandwidthof the linear region was approximately 50 MHz, which corresponds to the strain of up to 1035 "
and the temperature change of ∼45 K. In this linear region, the theoretical strain and temperature dependence coefficients were calculated to be 2:11  104 dB=" and 4:27  103 dB/K, respectively.
Note that these coefficients will be different if the experimental conditions alter. For instance, if thespatial resolution is set lower, the coefficients will be larger, while if the video bandwidth (VBW) of anelectrical spectrum analyzer (ESA) is set lower, they will be smaller.

3. Experimental Setup
The FUT employed in the experiment was a 5.0-m-long silica single-mode fiber (SMF) with a
BFS of 10.89 GHz at 1.55 m at room temperature. The experimental setup of SA-BOCDR isVol. 8, No. 3, June 2016

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Fig. 3. Experimental setup of SA-BOCDR. AMP, amplifier; EDFA, erbium-doped fiber amplifier;
ESA, electrical spectrum analyzer; FUT, fiber under test; LD, laser diode; OSC, oscilloscope; PD,
photo diode; PSCR, polarization scrambler.

schematically shown in Fig. 3, which is basically the same as that of conventional BOCDR[23], [25]; the only essential difference lies in the final signal processing. The output light froma distributed-feedback laser diode at 1.55 m was divided into two light beams, pump and reference. The pump light was amplified to ∼24 dBm using an erbium-doped fiber amplifier (EDFA)
and injected into the FUT. After passing through a ∼1-km-long delay line and another EDFA
(amplified to ∼5 dBm), the reference light was used for heterodyne detection with the Stokes
light, which was amplified to ∼2 dBm. The polarization-dependent fluctuations were suppressedusing a polarization scrambler. The heterodyned optical signal was converted into an electricalsignal using a photo diode and was guided to an ESA (VBW: 3 kHz, resolution bandwidth:
300 kHz). Using the narrowband-pass filtering function of the ESA, the PB0 change at a fixed
frequency B0 (= 10.85 GHz) was sequentially output to an oscilloscope (OSC).
A 0.1-m-long section around the distal open end of the FUT was bent to suppress the Fresnel reflection. The modulation frequency fm and amplitude f were set to 7.975–8.055 MHz and 1.4 GHz,
respectively, corresponding to the measurement range of 12.9 m and the theoretical spatial resolution of 88 mm from (1) and (2). The repetition rate was 100 Hz, and 16 times averaging was performed on the OSC. The room temperature was 26 °C.

4. Experimental Results
First, we investigated the PB0 change dependence on strain. Strains of 0 to 850 " were applied
to a 0.2-m-long section (3.5–3.7 m away from the circulator) of the FUT. The measured PB0change distributions along the FUT are shown in Fig. 4(a). With increasing strain, the PB0change increased (PB0 itself decreased). The PB0 change dependence was almost linear in thisrange [see Fig. 4(b)], and its coefficient was 1:95  104 dB=", which moderately agrees withthe theoretical value ð2:11  104 dB="Þ. Note that, in Fig. 4(a), the SNR was so low that relatively small strains of G 300 " were unable to be distinguished from the signal fluctuations inthis measurement.
Subsequently, the PB0 change dependence on temperature was measured. The result obtained when the temperature was locally changed to 75 °C in a 0.2-m-long section (2.0–2.2 m)
is shown in Fig. 4(c). With increasing temperature, the PB0 change increased. The PB0 change
dependence was almost linear [see Fig. 4(d)] with a coefficient of 4:42  103 dB/K, which
agrees with the theoretical value ð4:27  103 dB/KÞ.
The loss dependence of the PB0 change distribution was also measured. Bending losses of 0
to 2.8 dB were applied at a midpoint of the FUT (2.5 m away from the circulator). As shown in
Fig. 5(a), with increasing loss, the PB0 change increased on the distal side from the loss-applied
point (again, note that the PB0 value itself decreased). The loss dependence of the PB0 change[averaged in the 2.5-m-long distal section (2.5–5.0 m)] was almost linear [see Fig. 5(b)] with acoefficient of 0.191.
We then evaluated the stability of this method. When neither strain/temperature change nor loss
was applied to the FUT, we measured the temporal variations of PB0 for 20 ms (see Fig. 6). The

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Fig. 4. (a) PB0 change distribution when strains were locally applied. (b) PB0 change dependence
on strain. The dotted line is a linear fit. (c) PB0 change distribution when temperature was locallychanged. (d) PB0 change dependence on temperature. The dotted line indicates a linear fit.

Fig. 5. (a) PB0 change distribution when losses were locally applied. (b) PB0 change dependence on
loss. The dotted line is a linear fit.

standard deviation was calculated to be ∼0.0178 dB, which corresponds to a strain error of
þ=  92 ", a temperature error of +/− 4 K, and a loss error of +/− 0.1 dB. These values, which
can be suppressed further by averaging more data at the cost of a reduced sampling (or repetition)rate, showed no quantifiable changes when the measurement time was longer than 10 minutes.
Finally, a proof-of-concept demonstration of SA-BOCDR was performed by simultaneous measurement of strain, temperature, and loss. The structure of the 5.0-m-long FUT is shown in Fig. 7(a);ambient temperature was changed to 55°C along the 1.9–2.1-m section, a 0.64 dB loss was applied at the midpoint, and a 550 " strain was applied to the 3.5–3.7-m section. Fig. 7(b) shows
the measured PB0 change distribution along the FUT. The PB0 changes corresponding to thetemperature change, loss, and strain were observed at the expected sections. The amounts ofVol. 8, No. 3, June 2016

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Fig. 6. Measured temporal variations of PB0 .

Fig. 7. (a) Structure of the FUT. (b) Measured PB0 change distribution.

the PB0 changes for the temperature change, loss, and strain were approximately 0.16, 0.14, and0.12 (= 0.26–0.14) dB (corresponding to the temperature change, loss, and strain of ∼62 °C,∼0.73 dB, and  615 ", respectively). The measurement errors probably originate from thesignal fluctuations and the resultant low SNR, which needs to be improved by optimal low-passfiltering and/or increase in the number of averaging in future.

5. Conclusion
We experimentally proved the concept of a new BOCDR configuration, named SA-BOCDR,
which can perform the simultaneous distributed measurement of strain (or temperature) and optical loss by exploiting the slope of the BGS. After measuring the strain-, temperature-, and lossdependence coefficients of the output signal (1:95  104 dB=", 4:42  103 dB/K, and 0.191,
respectively), we verified the basic operation of simultaneous measurement of the three parameters. The improvement of the low SNR is one of the most important future tasks. The limitationof the system performance, such as strain/temperature/loss dynamic ranges, measurement accuracy, and repetition rate, also needs to be clarified.

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